kde-workspace: Disable parallel building
kde-workspace: Disable parallel building

Maybe this fixes this build failure:

In file included from /tmp/nix-build-103m3iz49n04a7h1pzinalqnw7x5g0da-kde-workspace-4.8.4.drv-0/kde-workspace-4.8.4/plasma/generic/runners/kill/killrunner.h:28:0,
                 from /tmp/nix-build-103m3iz49n04a7h1pzinalqnw7x5g0da-kde-workspace-4.8.4.drv-0/kde-workspace-4.8.4/plasma/generic/runners/kill/killrunner.cpp:20:
/tmp/nix-build-103m3iz49n04a7h1pzinalqnw7x5g0da-kde-workspace-4.8.4.drv-0/kde-workspace-4.8.4/plasma/generic/runners/kill/killrunner_config.h:24:34: fatal error: ui_killrunner_config.h: No such file or directory
compilation terminated.

http://hydra.nixos.org/build/2837527

NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,triton/triton,triton/triton,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs